The fluidized bed technology in olefin polymerization reactors used today can be adjusted to produce a wide variety of products. This is particularly true for polyethylene manufacture. Generally, one reactor system must produce resins that can be used in a variety of applications including injection molding, blow molding, roto-molding, wire coating, piping and tubing manufacture, and film manufacture. Basic fluidized bed reactor technology is employed to make a wide variety of polyolefin products, e.g., homopolymers and copolymers of polyethylene, polypropylene, C.sub.4 -C.sub.12 alpha olefins; ethylene-propylene-diene monomer (EPDM), polybutadiene, polyisoprene, and other rubbers. In general, polymer products are made using the same reactants in the same reactor, but reactor conditions such as reactant ratios and operating temperatures determine final properties of the polymer products. In addition, polymer products are produced with a number of different resin properties, or grades, wherein each grade of polymer product has a narrow range of properties, e.g., density and melt index.
The length of time a reactor is used to make a particular type of polymer product depends on the market demand for the product. Demand for some products are so high that a reactor can continue producing the same product for weeks without change. Other products, because of their lower market demand or availability of reactant, are made for much shorter periods of time. Unfortunately, industrial reactors require transient time to adjust to the new conditions (e.g., temperature, reactant pressures, and reactant ratios). Material produced during this transient or interim time is constantly changing but generally does not fall within the properties (e.g., melt index and density) of either the old product or the new product to which the reactor is being adjusted to during the transition. Changing the mode or operation of a reactor from one product to another product is not made instantaneously and requires a quantifiable and significant period of transiency time, or transient time, to become adjusted to the desired conditions for producing the new product. Similarly, fluidized reactors operating at desired or fixed conditions, i.e., at "steady state", can experience fluctuations that can result in the production of "offgrade" material. This production of offgrade material represents an economic loss and is desirably minimized.
Generally, industrial control systems for gas phase, fluidized bed polymerization reactors are designed to permit the operators to control the reactor by allowing the operators to select a desired target property such as a melt point index and a product density. Correlations of these properties relating the property to reactor processing variables are usually well known by the operators and those in the art for the particular reactor design and catalyst being used. The prior art has devised a number of methods to reduce the transient, offgrade material. These methods typically involve some combination of adjusting the automatic flow/ratio controllers to a new value either at or above the ultimately desired value ("dial-in transition" and "overshoot"), removing the reactant gas entirely ("inventory blow down"), reducing the level of the catalyst ("low bed"), and adding a nonreactive gas ("nitrogen addition").
DE-4241530 describes using a kill gas to stop a polymerization reaction, blowing the gas inventory for that reaction out of the reactor, and rebuilding a new gas inventory for a new product. This method reduces transition material. The costs associated with throwing away the old gas inventory and rebuilding a new inventory are too high for commercial transitions between closely related grades. Thus, most transitions between grades of the same material are performed by adjusting the reaction conditions.
McAuley et al. ("Optimal Grade Transitions in a Gas Phase Polyethylene Reactor", AIChE Journal, October 1992, Vol. 38, No. 10, pp. 1564-1576) discloses three manual, labor-intensive transition strategies for gas phase polyethylene reactors. The first is an adjustment to the controls to overshoot the melt index and density values. The hydrogen feed and co-monomer feeds are increased to meet the designated properties. The actual desired setpoint values are directed when the sensors indicate that the desired product is being produced. The second is an increase in temperature and manipulation of the slow vent to move the melt index of the produced product. The third is a drop in the catalyst level of the lower bed while keeping the bed resin residence time at a constant to reduce offgrade production.
Debling, et al., ("Dynamic Modeling of Product Grade Transitions for Olefin Polymerization Processes", AICHE Journal, March 1994, Vol. 40, No. 3, pp. 506-520) compares transition performance of different types of polyethylene reactors. The article discloses seven separate manual, labor-intensive transition strategies: (1) dialing in the final aim transition; (2) gas inventory blow down and simple dial-in transition; (3) low bed and simple dial-in transition; (4) gas inventory blow down and overshoot of melt index and density transition; (5) low bed, gas inventory blow down, land overshoot transition; (6) low bed and overshoot transition; and (7) gas inventory blow down, overshoot, and nitrogen addition transition.
U.S. Pat. No. 5,627,242 discloses a method for controlling a gas phase polymerization reaction in a reactor when changing from a first product made at a first set of conditions to a second product made at a second set of conditions using a correlated melt index to adjust the property of the reaction product. New reactor control setpoints are established to adjust the approach toward the second set of conditions by offsetting the reactor temperature and reactant partial pressure according to the magnitude and direction of the difference between the first and second product melt indices.
Permeable membrane processes and systems are known in the art and have been employed or considered for a wide variety of gas and liquid separations. In such operations, a feed stream is brought into contact with the surface of a membrane, and the more readily permeable component of the feed stream is recovered as a permeate stream, with the less readily permeable component being withdrawn from the membrane system as a non-permeate stream. Membrane separation modules are maintained at operating conditions which result in a non-permeate side pressure at which the feed gas is introduced and the non-permeate stream is withdrawn, and a permeate side pressure at which the permeate stream is withdrawn. The pressure on the non-permeate side of the membrane is higher than the pressure on the permeate side, and the pressure differential between the non-permeate and the permeate sides of the membrane generally determines the degree of separation attained by the membrane separation.
Despite this wide variety of available schemes, there is a continuing need and desire to reduce the amount of offgrade material produced during transition to a new product grade or during steady-state manufacture.
It is an object of the invention to provide a method for reducing the amount of off-grade material produced during grade transition or during steady-state manufacture.
It is another object of the invention to provide a method for reducing the transition time and volume of transient material when switching from one polymer product to another product of similar chemistry but different properties.